Unusual presentation of chondroblastoma mimicking Trevor's disease.

Chondroblastoma is a benign bone tumor, represents 1%-2% of all primary bone tumors, typically seen in patients 10-25-year-old and more common in males. It occurs most frequently in the distal femur, proximal tibia, and proximal humerus. Soft tissue extension is extremely rare. Adjacent joints may develop effusions, but the tumor mass protruding into the joint has never been seen in case of chondroblastoma. We report a rare case of intra-articular chondroblastoma arising from proximal tibia in a 16-year-old boy and growing into the knee joint mimicking an intra-articular osteochondroma.